Java在运行时确定类型T

Question

I have the following Java code;

PagedResponse<Person> response = new PagedResponse<Person>();
TypedQuery<Person> query = getNamedQuery("Person.findSpecific", Person.class);
response = executePagedTypedQuery(query);

Now executePagedTypedQuery is defined as;

protected <T> PagedResponse<T> executePagedTypedQuery(TypedQuery query) {

PagedResponse<T> response = new PagedResponse<T>();
List<T> resultList = query.getResultList();

}

Now I want that inside executePagedTypedQuery() method, the type T should be set to "Person", which is what I am passing. But for some reason, on debugging, it says

T = >"T" is not a known variable in the current context.<

Am I passing the parameter incorrectly ?

Answer 1

If you change the signature to

protected <T> PagedResponse<T> executePagedTypedQuery(TypedQuery<T> query)

then when you pass in your query, T will be chosen accordingly.

Answer 2

You're not invoking the method incorrectly; T is not passed as a parameter to the method.

Generics in Java are basically compile-time syntactic sugar to prevent you having to do lots of casts everywhere. The compiler will generate the compiled code of the method assuming that all references to T are references to the maximal parent type T if it has a type constraint, or Object if it doesn't have a constraint. This is called type erasure .

Contract this with the situation in C# - type parameters are actually accessible to your code, so you can write things like new T(); or typeof(T) (equivalent to Java T.class ) in your method.